The range of uses, such as homes, offices, photographs and the outdoors, for ink-jet recording systems tends to increase and importance has been placed on the weather (light) resistance and the storage property of printings. To date, dyes have been in dominant positions as colorants for ink-jet recording but have many problems in the light resistance. Along with that, an ink-jet recording aqueous ink exhibiting excellent light resistance has been developed.
The ink used for the ink-jet recording system is ejected from a nozzle and, therefore, is required to have both good quality intrinsic to the ink and good print quality. The intrinsic quality includes that the viscosity is low, precipitation or coagulation due to no use for a long time does not occur, i.e. good storage stability is exhibited, and plugging does not occur in an end portion, and an ink flow passage of an ink-jet printer head.
In order to obtain the ink quality such that the viscosity is low, good storage stability is exhibited, and plugging does not occur, in general, an ink, in which a pigment is stably dispersed in water or an aqueous medium by a surfactant, a high-molecular-weight dispersing agent, or the like so as to minimize the average diameter and the pigment concentration is usually adjusted to 10% or less, is used.
Among them, an ink-jet recording aqueous black ink that uses a black pigment, e.g., carbon black, is expected to substitute for electrophotography in homes and offices. However, as described above, the ink in the ink-jet recording system has problems that the optical density (also referred to as OD value, image density, or coloring density) at the time of printing is low because printing is performed in the state, in which a liquid has a pigment concentration much lower than the pigment concentration of the toner, and in addition, the ink permeates the paper easily and a high optical density is not obtained easily in the case where the medium is plain paper.
On the other hand, in general, the ink is prepared by diluting a composition, which is called, a pigment paste or pigment dispersion, having a high pigment concentration with water or an aqueous medium. Therefore, investigations on the optical density were performed with respect to the ink having a low pigment concentration and, in addition, the pigment dispersion itself having a high pigment concentration.
For example, PTL 1 discloses an aqueous black pigment dispersion containing carbon black having [1] a primary particle diameter of 20 to 30 (nm), [2] a specific surface area of 100 to 150 (m2/g), and [3] DBP oil absorption of 40 to 120 (cm3/100 g) and a styrene-acrylic copolymer, in which [a] a polymerization unit of a benzyl (meth)acrylate-based monomer is contained, [b] the total amount of styrene-based monomer and benzyl (meth)acrylate-based monomer is 45 to 80 percent by mole, [c] the number of moles of styrene-based monomer to the total number of moles of styrene-based monomer and benzyl (meth)acrylate-based monomer is 65% to 95%, and [d] a styrene-acrylic copolymer having an acid value of 110 to 220 (mgKOH/g) is contained, as a pigment dispersion for a ink-jet recording aqueous pigment ink, which provides a high coloring density (OD value) on the plain paper.
Also, PTL 1 discloses that there is no particular limitation regarding the dispersing method, various known dispersing machines can be used, and media agitation type dispersing machines are preferable. In the example, dispersion is performed by a dispersing apparatus (SC Mill Model SC 100/32 produced by Mitsui Mining Co., Ltd.) filled with zirconia beads.
PTL 1 discloses that in order to obtain a high OD value, specific carbon black having predetermined particle diameter, specific surface area, DBP oil absorption, and the like is preferable. However, preferable surface properties are not indicated and there is no description on, for example, a method for adding the same resin having anionic groups for maintaining good storage stability.